Although non-A, non-B hepatitis agents cannot be detected by serologic means, they can be experimentally transmitted to chimapanzees and marmosets. These species have been useful in determining the infectivity titers of various non-A, non-B containing plasmas. Although most plasmas contain only 10 squared - 10 cubed infectious units per ml, one plasma was found to contain over 10 to the six infectious units per ml. This plasma have provided an inoculum suitable for characterization of the agent. We have recently demostrated that at least one non-A, non-B agent contains essential lipids, a characteristic that will be important in the classification and, probably, the control of non-A, non-B hepatitis.